The present invention relates to an electrophotographic apparatus wherein an electrophotographic photosensitive member is exposed to a laser beam modulated in accordance with information to be recorded, more particularly, to such an apparatus wherein a semiconductor laser and a photosensitive member containing, as a main content amorphous Si are used as the above laser and photosensitive member, respectively.
An electrophotographic photosensitive member containing as a main component CdS or amorphous Si does not have a sufficient sensitivity to the semiconductor laser beam having the wavelength larger than 700 nm.
Recently, a particular attention is directed to an electrophotographic photosensitive member containing as the main content amorphous Si. The amorphous Si has a sufficient sensitivity also to a long wavelength beam, such as a semiconductor laser beam. However, with repeated exposure thereof to the semiconductor laser beam of a long wavelength, a remarkable exposure hysteresis takes place in the amorphous Si, resulting in a deteriorated image record. More particularly, the recorded image involves, as a ghost image, the exposure hysteresis caused by the prior image information.
FIG. 1 shows an example of a spectral sensitivity of an amorphous Si, which almost all amorphous Si exhibit. FIG. 1 shows the relation between the wavelength of the exposure light and the amount of exposure required for decreasing the photosensitive member surface potetial from 400 V to a half thereof, that is, 200 V.
As will be understood from FIG. 1, with the wavelength longer than 760 nm, the photosensitivity becomes low. However, the amorphous Si photosensitive member still has a practically sufficient sensitivity to the semiconductor laser beam having the wavelength of 700 nm-800 nm.
FIG. 2 shows a test apparatus, to which the amorphous Si photosensitive member having the characteristics shown in FIG. 1 was incorporated. The photosensitive member in the form of a drum includes a conductive drum of aluminum and an amorphous Si layer having 20 microns thickness coated thereon, and is rotatable in the direction shown by an arrow at a peripheral speed of 200 mm/sec. Into the amorphous Si, a small amount of an impurity (hydrogen or nitrogen or the like) is doped. The photosensitive member 1 is first electrically charged to the positive polarity by a charger 2 supplied with 435 .mu.A current. Then, it is exposed to a semiconductor laser beam 3 having 785 nm wavelength. The amount of exposure is 4 J/cm.sup.2. The laser beam 3 is produced by a semiconductor laser LD and is focused as a spot on the photosensitive member 1 by a lens LS. The beam 3 is scanningly deflected in a direction substantially perpendicular to the rotation of the photosensitive member by a scanner SC, such as a polygonal mirror. The apparatus is equipped with a detector 4 for detecting the surface potential of the photosensitive member 1 and a discharger 5 for applying corona discharge of the negative polarity to the photosensitive member 1 to restore the surface potential of the photosensitive member 1 substantially to 0 V.
FIG. 3 shows the results of the test. During the first rotation of the photosensitive member 1, the semiconductor laser LD was kept off. By this rotation, the photosensitive member was charged by the charger 2 to 400 V (a.sub.1) of the surface potential. Then, the photosensitive member 1 was continuously rotated for three turns with the semiconductor laser LD kept on to expose it to the beam 3. The surface potential of the photosensitive member 1 after the exposure to the laser beam 3 is indicated for each of the rotations by the references b.sub.2, b.sub.3 and b.sub.4.
After the above 4 rotations of the photosensitive member, the semiconductor laser LD was kept off, again. Then, the surface potential of the photosensitive member 1, after subjected to the charger 2, changes with the rotation as shown by the references c.sub.5, c.sub.6, c.sub.7 and c.sub.8. The reference c.sub.5 depicts approximately 350 V of the surface potential of the photosensitive member 1. In this specification, the difference in the surface potential of the photosensitive member between the point a.sub.1 and the point c.sub.5 is called "exposure hysteresis". In the device shown in FIG. 2, the exposure hysteresis of approximately 50 V is exhibited, which is large enough to produce a remarkable ghost image. On the other hand, a copying machine wherein an amorphous Si photosensitive member is exposed to image light through a lens, which image light is obtained by illuminating a document with a fluorescent lamp or a halogen lamp, does not practically involve a sufficient amount of the exposure hysteresis to be a problem. However, when a semiconductor laser beam having a long wavelength is used, the amorphous Si photosensitive member involves the strong exposure hysteresis. It is thought that this is because the semiconductor laser beam having the long wavelength can reach deep into the bulk of the amorphous silicon, and a large amount of the electric charge is captured in deep traps. It is considered that, with a short wavelength beam, such a phenomena rarely occurs.